1. Field of the Invention
The present invention is related to a method for controlling exposure time of high dynamic range images, and particularly related to a method of using an image sensor to capture images of different exposure durations, merging images of different exposure durations to form a high dynamic range image, and adjusting an exposure duration ratio to obtain a high dynamic range image of best quality.
2. Description of the Related Art
The dynamic range of a contemporary consumer digital camera or webcam, which is the ratio between the greatest to the smallest brightness values, is around 400, and the dynamic ranges of more professional single-eye cameras can be increased up to 1000. However, most of the dynamic ranges of indoor scenes exceed 1500, while the dynamic ranges of outdoor scenes usually reaches up to 105. Therefore, the present equipments can merely capture partial brightness information of scene.
To solve the aforementioned problem, P. E. Debevec proposed the technology of the high dynamic range image to merge a plurality of images of different exposure durations in 1997 in “Paul E. Debevec, Jitendra Malik, Recovering High Dynamic Range Radiance Maps from Photographs, SIGGRAPH 1997”, namely combining a plurality of images having different exposure durations. Higher dynamic range information can be obtained by increasing the exposure duration ratio of two original images. However, it is also necessary to compress more dynamic range information into the displayable dynamic range of a screen, and this process is called color mapping or tone mapping. Since the dynamic range of a screen is very low and about 200, the compression process of color mapping will destroy details and contrast of an image and cause the overall image quality to downgrade. In other words, best image quality cannot be obtained by simply increasing the exposure duration ratio. Besides, most high dynamic range image systems in the market are outfitted with a fixed exposure duration ratio, or require a user to manually set up the exposure duration ratio.